You won't have any issues leaving your phone charging overnight. Years ago the technology within lithium ion batteries like you find I'm your phones, tablets, etc, advanced to the point that you can't overcharge it. There are recommended charge levels dependent on the temperature your device is exposed to. Though that's only if you're overly concerned with the longevity of your battery. Charging overnight or for multiple hours will not harm your battery and it will still last 2+ years. Assuming you're not using your phone so heavily that it needs a full charge 3-4 times a day.

Nope, nothing wrong with that. I did that with my androids, and I’m doing it with my iPhone. They’ve got sensors in them that signals for the charger to stop sending in charge to keep it from getting overcharges. That’s how Li-ion batteries work. It’s a safe proof.

No issues and modern batteries can last longer that five years. My mother’s i4 battery is weak but still original. Although she rarely uses her phone it still serves her needs but stays plugged in most of the time